Climate models are used to predict the impact of climate change. However, poorly constrained parameters used in the physics of the models mean that we simulate a large spread of possible future outcomes. We can use real-world observations to reduce the uncertainty of parameter values, but we do not have observations to reduce the spread of possible future outcomes directly. We present a method for translating the reduction in parameter uncertainty into a reduction in possible model projections.

While a 6-year oscillation has been reported for some time in the motions of the fluid outer core of the Earth, in the magnetic field and in the Earth rotation, novel results indicate that the climate system also oscillates at this 6-year frequency. This strongly suggests the existence of coupling mechanisms affecting the Earth system as a whole, from the deep Earth interior to the surface fluid envelopes.

We analyse the duration of large-scale patterns of air movement in the atmosphere, referred to as persistence, and whether unusually persistent patterns favour warm-temperature extremes in Europe. We see no clear relationship between summertime heatwaves and unusually persistent patterns. This suggests that heatwaves do not necessarily require the continued flow of warm air over a region and that local effects could be important for their occurrence.

Nitrogen (N) is an essential limiting nutrient to terrestrial carbon (C) sequestration. We evaluate N cycling in an ensemble of terrestrial biosphere models. We find that variability in N processes across models is large. Models tended to overestimate C storage per unit N in vegetation and soil, which could have consequences for projecting the future terrestrial C sink. However, N cycling measurements are highly uncertain, and more are necessary to guide the development of N cycling in models.

The change in extreme-event occurrence is influenced by both a shift in the mean and a change in variability. How large the individual contributions are remains largely unknown. Large-ensemble climate simulations and probability risk ratio are used to partition the change in extreme precipitation events into contributions from a change in the mean and variability. The results reveal that the change in variability can be equally as important as or even more important than the mean change.

There are many possibilities for future emissions, with different impacts in different places. Complex models can study these impacts but take a long time to run, even on powerful computers. Simple methods can be used to reduce this time by estimating the complex model output, but these are not perfect. This study looks at the accuracy of one of these techniques, showing that there are limitations to its use, especially for low-emission future scenarios.

A global climate model is applied to quantify the impact of land transport, shipping, and aviation on aerosol and climate. The simulations show that these sectors provide relevant contributions to aerosol concentrations on the global scale and have a significant cooling effect on climate, which partly offsets their CO₂ warming. Future projections under different scenarios show how the transport impacts can be related to the underlying storylines, with relevant consequences for policy-making.

The second law of thermodynamics has long intrigued scientists, but what role does it play in the Earth system? This review shows that its main role is that it shapes the conversion of sunlight into work. This work can then maintain the dynamics of the physical climate system, the biosphere, and human societies. The relevance of it is that apparently many processes work at their limits, directly or indirectly, so they become predictable by simple means.